The present invention relates generally to methods and apparatuses for the electrolytic reduction of alumina to aluminum. More particularly, the subject matter herein is related to the subject matter disclosed in the following U.S. Pat. Nos. of Beck et al.: 4,592,812, 4,865,701, 5,006,209 and 5,284,562, and the disclosures thereof are incorporated herein by reference.
The aforementioned patents of Beck et al. are directed to a series of developments relating to the electrolytic reduction of alumina to aluminum. The developments culminated in an electrolytic reduction cell containing a relatively low melting point, molten electrolyte composed of fluorides, a non-consumable anode composed of a particular alloy of copper, nickel and iron, and a cathode, composed of titanium diboride (TiB.sub.2), that is wettable by molten aluminum. A plurality of the non-consumable anodes are vertically disposed within a vessel containing a bath composed of molten electrolyte. A plurality of the cathodes are also vertically disposed within the vessel, with the cathodes being arranged in close, alternating, spaced relation with the vertically disposed anodes. In a preferred embodiment, the vessel has an interior metal lining electrically connected to the anodes and having essentially the same composition as the anodes; the lining functions as an auxiliary anode.
The bath of molten electrolyte contains some dissolved alumina and is saturated with additional alumina in the form of finely divided particles. The molten electrolyte has a density less than the density of molten aluminum and less than the density of alumina. As noted above, some alumina is dissolved in the molten electrolyte. When an electric current is passed through the bath, aluminum ions are attracted to the cathodes, and oxygen ions are attracted to the anodes. Bubbles of gaseous oxygen form at each of the anodes, and metallic aluminum forms at each of the cathodes. The bubbles of gaseous oxygen pass upwardly from the anodes and maintain the undissolved, finely divided alumina particles suspended in the bath of molten electrolyte, forming a slurry. The metallic aluminum formed at the cathodes wets the surface of each cathode and flows downwardly along the cathode.
The electrolytic reduction cell is operated at a relatively low temperature, substantially below 950.degree. C. The composition of the electrolyte employed in the cell enables one to operate the cell at a relatively low temperature, because the electrolyte is molten at that low temperature. The low cell temperature allows one to use non-consumable anodes composed of the Ni-Cu-Fe alloys described below without subjecting the anodes to deterioration in the molten electrolyte. However, when the cell is operated with a molten electrolyte having a temperature below about 800.degree. C., certain problems can occur. Low solubility of aluminum oxide can lead to a deficiency of aluminum ions at the cathodes, causing a localized change of bath composition and cathode deposits. The deposits form on the surface of the titanium diboride cathode; these deposits interfere with the wetting of the cathode surface by metallic aluminum. In addition, a thin deposit forms around balls containing metallic aluminum that has separated from the cathode preventing the aluminum in these balls from agglomerating into a continuous phase of molten aluminum. Instead, the balls, containing metallic aluminum that has been coated with deposit, remain suspended within the molten electrolyte. The balls that discharge back into the bath back-react, causing poor current efficiency.
The deposits on the cathode are (a) cryolite (25 mol % NaF-75 mol % AlF.sub.3) or (b) cryolite and a suboxide of aluminum together with alumina. The deposit on the small balls containing aluminum is believed to be electrolyte and/or alumina. The root cause of the deposits on the cathode and on the balls containing metallic aluminum is believed to be insufficient dissolution of alumina in the electrolyte, particularly at locations adjacent the cathodes.
On start-up, the cathode deposits can form quickly. Once formed, the deposits can persist throughout a run with poor aluminum recovery and low current efficiency. The deposit that appeared to be cryolite was an even grey deposit that would not melt at low operating temperatures. Cryolite deposits can be formed with higher current densities, a low concentration of aluminum fluoride, and coarse alumina feed.
A lumpy, black deposit formed at modest current densities. This deposit has a composition not unlike the bath composition. The black color may be from the formation of a suboxide of aluminum, which is a black semiconductor. Roasting these deposits in air at higher temperatures turns them white. Inspection of these deposits with x-ray fluorescence shows them to be similar to the bath in composition. Cathode deposits are accompanied by a grey bath color, which appears to be associated with aluminum discharge back into the bath from the non-wetted cathode.
Initial deposits may be formed by metal sulfides formed from aluminum sulfate or other sulfur compounds. Aluminum sulfate is an impurity in some smelter grade aluminum fluoride and it enters the bath with the aluminum fluoride feed. Sulfate ion is reduced by aluminum to sulfides, and perhaps to SO.sub.2, SO.sub.3, and sulfur. Soon after reduction of the sulfate ion, the bath is purged of sulfur.
Known expedients for increasing dissolution of alumina in the electrolyte can reduce or eliminate formation of the deposits, but these known expedients have drawbacks. More particularly, one known expedient is to increase the temperature of the electrolyte substantially above 800.degree. C.; however, this expedient has the drawback of causing the anodes to deteriorate more rapidly.
Another known expedient is to increase the proportion of alumina dispersed in the molten electrolyte; however, an increase in the proportion of alumina increases the voltage required to operate the cell, and this in turn increases energy consumption which is undesirable.
Yet another approach is to pick an intermediate bath temperature, increasing alumina solubility, but also increasing electrode corrosion rate.
Still another known expedient for increasing the dissolution of alumina in the electrolyte is to reduce the current density (defined below), but this compromises the small footprint advantage obtained by employing a multiplicity of vertically disposed, alternating, spaced apart anodes and cathodes in the electrolytic reduction cell.
More particularly, current density is directly proportional to current and inversely proportional to the area of the anode which faces the cathode. The conventional electrolytic reduction cell employs a single horizontally disposed anode and a single horizontally disposed cathode. In order for a cell having a single horizontally disposed anode and cathode to provide a given, relatively low, current density, the required area of the facing surfaces of the two electrodes must be relatively large, and the cell will accordingly occupy a relatively large horizontal area or footprint. When one employs a multiplicity of vertically disposed, alternating, spaced apart anodes and cathodes, the same required area for the electrodes is distributed among the multiplicity of anodes and cathodes. Because the electrodes are vertical and closely spaced, together they occupy a relatively small horizontal area; accordingly, a cell containing such a vertically disposed arrangement of electrodes has a relatively small footprint.
In order to reduce the current density in a cell comprising the vertically disposed electrodes described above, one must increase the number of electrodes in the cell. This enlarges the cell and its footprint, thereby compromising the small footprint advantage, described above.